1. Field of the Invention
This invention relates generally to the field of wireless electronic equipment design and, more particularly, to radio frequency (RF) receiver design.
2. Description of the Related Art
A traditional narrow band radio system's receiver portion is generally used for translating a modulated radio frequency (RF) carrier to a low frequency, or baseband, signal, which may then be demodulated to extract an original modulating signal. A primary function of an RF receiver is best described as a “down conversion”, or “mixing”, of an RF modulated carrier to a baseband signal, and a filtering of the down converted signal to remove any artifacts that may have arisen as part of the mixing process. Due to the image effects that may occur during the mixing process, performing down conversion may be handled in multiple stages where two or more down conversion stages may be applied in series. A traditional super-heterodyne receiver often uses two down conversions stages separated by channel filtering to remove any effects from image frequencies. These channel filters have traditionally been implemented with external components due to the frequency and accuracy requirements of the receiver. Often some additional gain is also used before down converting the input RF signal as the mixing process typically adds noise to the baseband signal.
Down conversion mixers have traditionally been implemented with Gilbert mixers (as described in U.S. Pat. No. 4,156,283), which are voltage input, voltage output circuits that require large supply voltages. A large supply voltage limits the ability of RF receivers to operate from portable supply voltages, for example a battery, which limits their ease of use. Additionally, low noise amplifier (LNA) gain stages used at the input of RF receivers generally require large supply voltages in order to maintain proper isolation from their inputs.
One RF receiver topology that has generated significant interest and research activity is “direct conversion” receivers. Direct conversion receivers typically mix the modulated input RF signal directly to a baseband signal with a single mixing stage. The advantage of this topology is that the image-reject filters of the super-heterodyne receivers are no longer required, which generally translates into reduced cost. Disadvantages of direct conversion receivers include non-ideal effects like DC offsets in the baseband signal and self-mixing with the local oscillator to the RF input due to finite isolation. In the past, these problems were generally difficult to overcome, which is one main reason why a majority of RF receivers have been designed as a variation on the super-heterodyne architecture.
One example of a direct conversion receiver design is found in “A 2 GHz Wide-Band Direct Conversion Receiver for WCDMA Applications”, IEEE JSSC vol. 34, no. 12 December 1999. The receiver described in the above cited publication uses a voltage mode LNA at the front end, a Gilbert cell down-conversion mixer, and a voltage mode Butterworth channel selection filter. Typically, most direct conversion receiver architectures use at least one stage of voltage mode circuitry. Generally, direct conversion receivers to this point have relied on tried and true LNA and mixer topologies that are inherently voltage mode circuits. These architectures typically require significantly higher supply voltages than alternative current mode architectures, especially in case of Complementary Metal-Oxide Semiconductor (CMOS) circuits.